1. Field of the Invention
The present invention relates to an electric-power saving type high-performance, indirectly-heated cathode assembly for use, for example, in a color CRT (cathode ray tube) and its associated electron gun tube structure.
2. Description of the Related Art
Recently, there is a growing demand for a color CRT of an improved resolution with added scanning lines, an ultrahigh frequency-responsive picture tube and so on. A demand is also made for improved brightness, for example, in a projection CRT. In order to meet these demands, the density of emission electron from the cathode need to be increased to a greater extent.
An emitter-impregnated type cathode can obtain a greater current density than an oxide cathode. For this reason, the emitter-impregnated type cathode has been employed for a pickup tube, travelling-wave tube, Klystron and so on. In the field of color CRTs, however, the emitter-impregnated type cathode finds only a limited application.
The emitter-impregnated cathode of indirectly heated cathode assembly is constructed, such a type as shown in FIG. 1. In the structure shown in FIG. 1, a heater 1 is located within a cathode sleeve 2. A cap 4 is fitted into one end of the cathode sleeve 2 and has an emitter-impregnated cathode disc 3. A cylindrical holder 6 is disposed outside the cathode sleeve 2 such that it is situated coaxial with the cathode sleeve 2. The cathode sleeve 2 is fixedly supported by three straps 5 made of tantalum.
The operation temperature of the aforementioned indirectly heated cathode assembly is higher than that of the oxide cathode type by about 200.degree. C. Thus the indirectly heated cathode assembly requires more heater's electric power, presenting a bar to its practical application.
For economy in the electric power of the indirectly heated cathode assembly, it is necessary that it be made compact. In order to obtain a compact unit, it will be proved effective to reduce the cross-sectional area of the strap and the heat conduction loss.
However, the straps are so employed as to support the cathode and, if made too small, will be deformed at the operation of the cathode due to a fatigue resulting from heat. As a result, the characteristics of the color CRT become defective, such as degraded brightness or color drift.
Japanese Utility Model Publication (KOKOKU) 59-33146 discloses a heat reflective means which is provided outside straps. In the structure of KOKOKU, the means is placed outside of straps and thermally contacted with straps, failing to achieve a saving in electric power and a compactness.
The Japanese Utility Model Publication (KOKOKU) 57-26514 also discloses a heat reflecting cylinder which is located between a sleeve and straps and fixed to the sleeve. Since, however, the heat reflecting cylinder is placed in direct contact with the sleeve, heat is dissipated through the sleeve during operation, failing to achieve a saving in electric power.